1. Field of the Invention (Technical Field)
The present invention relates apparatus and methods for measuring the amount of particulate matter in liquid feed streams, and more particularly to apparatus for measuring the rate at which particulate matter in an aqueous feed stream will clog or foul filters, specifically reverse osmosis membranes.
2. Background Art
Due to the increasing shortfall in fresh water supplies, the use of reverse osmosis (RO) systems to desalinate salt and brackish water has been on the rise. In concept, RO is a simple process. Water is forced through a membrane under pressure. The membrane rejects both dissolved and suspended solids producing a very pure permeate. The process may be described as filtration on a molecular or ionic level. Unlike most filtration processes, however, RO is not simple to monitor. Of particular importance is the need to monitor the feedwater going to the RO unit to determine its potential of clogging or fouling the membrane surface.
Monitoring the fouling tendency of an RO membrane is a challenge. Fouling tendencies of the feedwater are usually not noticed until the RO membranes are in need of cleaning. This results in expensive downtime or, worse yet, membrane replacement. Numerous methods have been used to measure the fouling tendency of feedwaters. These include turbidity, particle counting, and silt density index (SDI). It is difficult to correlate turbidity and particle counting to membrane fouling since they do not directly measure the fouling or "plugging" nature of the particles in suspension. SDI has proven useful in indicating fouling tendencies, however, the manual SDI method is tedious and time-consuming.
The American Society for Testing and Materials (ASTM) has published procedures for a simple test to determine the silt density index (SDI) of RO feedwaters. The ASTM test involves placing a 47 mm filter disk (with 0.45 micron pore size) in a filter holder. The feedwater is passed through the filter at 30 psi. The amount of time required (t1) for the first 500 ml of feedwater to pass through the filter is recorded. The feedwater is allowed to continue to pass through the filter at 30 psi for 15 minutes. At the end of the 15 minute period, the time required (t2) for another 500 ml of water to pass through the filter is recorded. The SDI is calculated using the following equation: EQU SDI=(100.times.(1-(t1/t2)))/T
Where:
t1=elapsed time for first 500 ml PA1 t2=elapsed time for last 500 ml PA1 T=15 minutes
The SDI value will range from 0 to 6.7. Any value less than 4 is considered suitable for RO feed from a membrane fouling standpoint.
Although it may seem archaic, the SDI test is still recognized as one of the best ways to predict the fouling potential of a feedwater on RO and nanofiltration membranes. Drawbacks to the conventional way of measuring SDI is the fact that it is a manual test requiring an operator's undivided attention, and the method is time consuming, requiring 30 minutes or more per analysis. Most RO facilities are fortunate if the SDI of the RO feedwater is checked once per day.
Automated SDI monitors developed to date also have drawbacks. Eisenhauer et al., U.S. Pat. No. 4,554,822 requires complicated equipment for the handling of filters in the form of rolls for the replacement of the filter after each measurement. Also, this automated SDI monitor still requires 20 minutes for data collection, and provides no information on the nature of the particulates within the feedwater that will eventually plug or foul RO membranes. Kaakinen, U.S. Pat. No. 5,253,514, appears to solves these problems by measuring at constant pressure the change in flow rate (Q.sub.t) of the feedwater through the filter at specific time intervals. This system allows a SDI measurement to be obtained in about 5 minutes. In contrast, the present invention has the capability of obtaining a SDI measurement every few seconds, and thus allows for virtually continuous monitoring of particulates in the feed stream. The present invention stores this data and/or uses the data to calculate a real-time fouling rate at the RO membrane.
One disadvantage of the Kaakinen device is that it does not provide for the change in zeta potential as particulates build up on the test filter. Many cases have been documented in which the conventional SDI test was not effective in identifying the fouling potential of RO feedwater. This is due, in large part, to the fact that the conventional SDI test does not simulate the chemical changes that occur in RO systems. As feed water permeates through the membrane, the dissolved solids concentration increases in the boundary layer next to the membrane. Depending upon feedwater quality, changes in hardness, salinity, and pH may also occur. These changes modify the electrostatic charges (zeta potential) which keep small particles suspended thus allowing them to coagulate and foul the membrane and membrane feed spacer. The present invention has the capability of solving this problem by adding at least one chemical reagent up stream of the test filter so that the zeta potential at the test filter more accurately correlates with the zeta potential at the surface of the RO membrane. The similarity in the zeta potential at the test filter and the RO membrane allows the system to more accurately monitor the fouling conditions at the RO membrane.